1. Field of the Invention
The present invention relates to a thermal printer, a thermal printing method, and thermosensitive recording material. More particularly, the present invention relates to a thermal printer and a thermal printing method, in which application of heat to thermosensitive recording material can be suitably managed in view of high quality in image reproduction, and thermosensitive recording material.
2. Description Related to the Prior Art
A thermal printer is known in the art of image forming devices. A thermal head is incorporated in the thermal printer, is constituted by a heating element array of numerous heating elements arranged in a main scan direction. A thermosensitive recording material is moved in a sub scan direction perpendicular to the main scan direction, while an image is recorded to the recording material line after line. The thermal head pressurizes the recording material, and applies heat to the same, to develop color at a desired density. There is a platen roller opposed to the thermal head. The recording material is subjected to the thermal recording while nipped between the thermal head and the platen roller.
JP-A 9-272217 discloses a type of the thermal printer capable of full-width printing in which an image is recorded to the recording material in its full width without keeping a blank margin region on any of peripheral portions of the recording material. Although the recording material in use for the full-width printing has a standardized size, an error is likely to occur in the size due to a tolerable range in the precision of the slitting or cutting. In view of this situation, the heating element array in the thermal printer has a greater length than is sufficient for the standardized width of the recording material. Also, a sensor is used for detecting a lateral edge of the recording material, to specify a position in the heating element array associated with the lateral edge.
However, the recording material is squeezed between the thermal head and the platen roller. The recording material has a pair of lateral edges extending in the sub scan direction and opposed to one another. On the lateral edges in particular, heat from the thermal head is applied to the recording material in a locally concentrated manner on a tapered end of the lateral edges. There occurs a problem of locally high density and scorch due to overheat in a very small area of the tapered end on the lateral edges.
In the technique of the thermal recording, heat energy applied by the thermal head to the recording material changes if a printhead temperature of the thermal head changed incidentally. This causes changes in the density of a recorded image. JP-A 7-061020 discloses a construction for preventing such changes in the density due to changes in the temperature. The printhead temperature of the thermal head is measured, so as to control an interval of printing of the thermal head according to the printhead temperature. Thus, heat energy applied by the thermal head to the recording material is corrected.
However, the temperature of the recording material also influences the heat energy applied to the recording material. There occurs a problem in low quality in recording an image because of the influence of the temperature of the recording material. There is no known technique of considering the temperature of the recording material for keeping quality of image recording.